Method for heating process gases for direct reduction systems

ABSTRACT

A method for reducing iron ore in the direct reduction method, in which the iron ore to be reduced is conveyed through a reduction unit such as a reduction shaft and is brought into contact with a reduction gas; the reduction gas is brought into the reduction unit and flows through the unit; after flowing through the unit, it is taken from the unit; after exiting the unit, the gas is prepared and possibly enriched with new gas components and is fed back again; and the generated gas is heated before entry into the reduction unit, characterized in that the heating of the reduction gas prior to the entry into the unit is carried out in an electrical fashion.

FIELD OF THE INVENTION

The invention relates to a method for heating process gases for directreduction systems.

BACKGROUND OF THE INVENTION

Steel production is currently carried out in a variety of ways. Classicsteel production is carried out by producing pig iron in the hot furnaceprocess, primarily out of iron oxide carriers. In this method, approx.450 to 600 kg of reducing agent, usually coke, is consumed per metricton of pig iron; this method, both in the production of coke from coaland in the production of the pig iron, releases very significantquantities of CO₂. In addition, so-called “direct reduction methods” areknown (methods according to the brands MIDREX, FINMET, ENERGIRON/HYL,etc.), in which the sponge iron is produced primarily from iron oxidecarriers in the form of HDRI (hot direct reduced iron), CDRI (colddirect reduced iron), or so-called HBI (hot briquetted iron).

There are also so-called smelting reduction methods in which the meltingprocess, the production of reduction gas, and the direct reduction arecombined with one another, for example the methods of the brands COREX,FINEX, HiSmelt, or HiSarna.

Sponge irons in the form of HDRI, CDRI, and HBI usually undergo furtherprocessing in electric furnaces, which is extraordinarilyenergy-intensive. The direct reduction is carried out using hydrogen andcarbon monoxide from natural gas (methane) and possibly synthesis gas aswell as coke oven gas. For example, in the so-called MIDREX method,first methane is transformed according to the following reaction:

CH₄+CO₂=2CO+2H₂

and the iron oxide reacts with the reduction gas, for example accordingto the following formula:

Fe₂O₃+6CO(H₂)=2Fe+3CO₂(H₂O)+3 CO(H₂).

This method also emits CO₂.

DE 198 53 747 C1 has disclosed a combined process for the directreduction of fine ores in which the reduction is to be carried out withhydrogen or another reduction gas in a horizontal turbulence layer.

DE 197 14 512 A1 has disclosed a power station with solar powergeneration, an electrolysis unit, and an industrial metallurgicalprocess; this industrial process relates either to the power-intensivemetal production of aluminum from bauxite or is intended to be ametallurgical process with hydrogen as a reducing agent in theproduction of nonferrous metals such as tungsten, molybdenum, nickel, orthe like or is intended to be a metallurgical process with hydrogen as areducing agent using the direct reduction method in the production offerrous metals. The cited document, however, does not explain this indetail.

WO 2011/018124 has disclosed methods and systems for producing storableand transportable carbon-based energy sources using carbon dioxide andusing regenerative electrical energy and fossil fuels. In this case, apercentage of regeneratively produced methanol is prepared together witha percentage of methanol that is produced by means of non-regenerativeelectrical energy and/or by means of direct reduction and/or by means ofpartial oxidation and/or reforming.

In the direct reduction method, the gas emerging downstream of thereduction shaft—after it is purified and the water has been separatedout and additional CO₂ separation in the HYL method or optionaladditional CO₂ separation in the HYL MIDREX method—is predominantly fedback into the process as recycling gas. As a rule, this gas is in turnenriched with natural gas in order to supply fresh reduction gas. In theHYL method, the gas, which the gas purification has cooled fromapproximately 105° C., is heated again to approximately 700 to 1100° C.and then a partial oxidation with oxygen is performed.

In the MIDREX method, CO₂ and water are transformed with natural gasinto H₂ and CO in a heated reformer in a temperature range fromapproximately 700 to 1100° C. Both methods share the fact that a partialflow of the gas that has been purified and is exiting the reductionshaft is introduced and is enriched with natural gas.

The reduction process can be expressed with the following equation:

Fe₂O₃+6CO(H₂)=2Fe+3CO₂(H₂O)+3CO(H₂)  (1)

In the MIDREX method, the following reactions take place in thereformer:

CH₄+CO₂→2CO+2H₂  (2)

CH₄+H₂O→CO+3H₂  (3)

In the HYL method, the following reaction takes place:

CH₄+½O₂→CO+2H₂  (4)

In both methods, the additionally used fossil fuel, namely natural gas,is used to heat the process gases and to heat the reformer.

One object of the invention is to create a method for heating processgases for direct reduction systems with which the heating of processgases can be better and more flexibly adapted to and optimized for anoverall process that is adapted to the energy demand and to theavailable energy.

Another object of the invention is to reduce CO₂ emissions.

SUMMARY OF THE INVENTION

In order to make the heating process more flexible, according to theinvention, the heating of the reduction gases and of the reformer ischanged to an electrical heating.

Preferably, the electrical energy can be produced from renewableresources, thus replacing fossil fuels.

This advantageously increases the flexibility of the process with regardto the energy sources used; this is achieved through combined heating bymeans of a variable use of fossil fuels and electrical energy.

In this regard, the invention has the advantage that electrical currentcan be considered to be 100% energy so that it can be completelyconverted into high temperature heat. The direct convertibility ofelectrical energy into heat permits the addition of a high degree offlexibility, particularly also with regard to the use of current peaksthat are inexpensively available on the market.

It is also advantageous that current from renewable energy sources suchas hydroelectric, wind power, or solar energy does not cause any CO₂emissions when it is produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained by way of example in conjunction withthe drawings. In the drawings:

FIG. 1 shows as an example the HYL Energiron method according to theprior art, with a natural gas-powered process gas heating;

FIG. 2 shows the HYL Energiron method according to the invention, withan electrically-powered process gas heating;

FIG. 3 is a very schematic depiction of the MIDREX method;

FIG. 4 is a very schematic depiction of an expensive and complexCO₂-optimized MIDREX method according to the prior art, with aCO₂-removal unit (e.g. VPSA—vacuum-pressure swing adsorption).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The HYL method is shown by way of example in FIG. 2 on the basis of acapacity of two million metric tons of direct reduced iron (DRI) peryear, including an electric arc furnace (EAF). The process gas from theshaft in which the iron ore is reduced is first conveyed through a waterseparation and then through a CO₂ separation. The circulating gas volumeflow in this case is approximately 500,000 m³ per hour. Approximately72,000 m³ of natural gas per hour is added to this gas flow, 56,000 m³of which is used for the reduction and approximately 16,000 m³ of whichis diverted for heating the process gas from 105 to 970° C. Next, oxygenis added to the heated process gas and this is then fed back into thereduction shaft.

In a method according to the invention (FIG. 2), the reduction gas islikewise taken from the shaft and conveyed through a water separationand a CO₂ separation. Thanks to the electrical heating of the processgas heating, it is only necessary to add a quantity of approximately56,000 m³ of natural gas per hour, which is split with oxygen into COand hydrogen in accordance with the above-mentioned formulas. The tablein FIG. 2 shows that this achieves a 21% reduction in CO₂ per ton ofreduced iron. In addition, because of the electric heating, the processcan be used in an exactly controllable and flexible way.

FIG. 3 shows the MIDREX method in which the exhaust gas is likewisewithdrawn in the reduction shaft and divided into a process gas flow anda heating gas flow. The process gas flow is conveyed through a processgas compressor until natural gas is added to it—particularly in a systemthat is likewise designed for 2 million metric tons of reduced iron peryear—in a quantity of approximately 63,000 m³ of natural gas per hour.This process gas passes through a heat exchanger, in which it ispreheated by the exhaust gases from the reformer to 600° C. and thenpasses through the reformer and in so doing, is heated to 980° C. and isconveyed back to the shaft as process gas, which is enriched withadditional natural gas and oxygen. The heating gas is likewise takenfrom the shaft furnace, enriched with natural gas, and conveyed into thereformer together with preheated combustion air. The total requiredquantity of natural gas is approximately 68,200 m³ per hour; by heatingthe reformer electrically, it is possible to compensate forapproximately 5,100 m³ of exhaust gas per hour with 52 Megawatts ofelectric power. As a result of this, it is possible on the one hand toachieve a 7.5% reduction of CO₂ per metric ton of reduced iron ore. Inaddition, this process can also be controlled in a more flexible,precise fashion thanks to the electric heating.

The invention has the advantage of achieving a simple and quicklyimplementable option for replacing fossil fuels with electrical powerfrom renewable energies. CO₂ emissions from direct reduction systems arealso reduced. The invention also makes it possible to successfullyoperate direct reduction systems in an effective and flexible way. Inparticular, in a steel production that is adapted to the availability ofregenerative energies with an electrically-powered preheating of processgas, particularly one with heating based on renewable energies, it ispossible to achieve an improvement and reciprocal adaptation.

It is also advantageous that such a system can inexpensively make use ofavailable current peaks.

1. A method for reducing iron ore in a direct reduction method,comprising: conveying the iron ore to be reduced through a reductionunit such as a reduction shaft and bringing the iron ore into contactwith a reduction gas; bringing the reduction gas into the reduction unitto flow through the unit; after flowing through the unit, taking thereduction gas from the unit; after exiting the unit, preparing the gasand possibly enriching the gas with new gas components and feeding thegas back again into the reduction unit; and heating the generated gasmixture or the reduction gas products from the generated gas mixture to700 to 1100 before entry into the reduction unit, wherein the heating iscarried out in a predominantly electrical fashion.
 2. The methodaccording to claim 1, comprising using electrical power fromregenerative energy sources for the electric heating.
 3. The methodaccording to claim 1, further comprising, after the gas has exited theunit, enriching the gas with natural gas, coke oven gas, or a synthesisgas from biomass or coal.
 4. The method according to claim 1, comprisingenriching the gas mixture with oxygen.
 5. The method according to claim1, comprising enriching the gas taken from the reduction shaft withnatural gas, coke oven gas, or a synthesis gas from biomass or coal andthen heating the enriched gas.
 6. The method according to claim 1,comprising enriching the gas taken from the reduction shaft with naturalgas, coke oven gas, or a synthesis gas from biomass or coal and thentransforming the enriched gas in a reformer.
 7. The method according toclaim 1, comprising ensuring a cost-optimized use of energy sourcesthrough a continuous evaluation of gas prices and electricity prices.